Ion-selective Janus membranes with one-way ionic transport in hypersaline solution approach efficient osmotic energy conversion.

The design and fabrication of a robust nanoporous membrane in large scale is still a challenge and is of fundamental importance for practical applications. Here, a robust three/two-dimensional polymer/graphene oxide heterogeneous nanoporous membrane is constructed in large scale via the self-assembly approach by chemically designing a robust charge-density-tunable nanoporous ionomer with uniform pore size. To obtain a nanoporous polymer that maintains high mechanical strength and promotes multifunctionality, we designed a series of amphiphilic copolymers by introducing a positively charged pyridine moiety into the engineered polymer polyphenylsulfone. The multiphysical-chemical properties of the membrane enable it to work as a nanogate switch with synergy between wettability and surface charge change in response to pH. Then we systematically studied the transmembrane ionic transport properties of this two-/three-dimensional porous system. By adjusting the charge density of the copolymer via chemical copolymerization through a controlled design route, the rectifying ratio of this asymmetric membrane could be amplified 4 times. Furthermore, we equipped a concentration-gradient-driven energy harvesting device with this charge-density-tunable nanoporous membrane, and a maximum power of ≈0.76 W m was obtained. We expect this methodology for construction of a charge-density-tunable heterogeneous membrane by chemical design will shed light on the material design, and this membrane may further be used in energy devices, biosensors, and smart gating nanofluidic devices.

Heterogeneous membranes composed of asymmetric structures or compositions have enormous potential in sensors, molecular sieves, and energy devices due to their unique ion transport properties such as ionic current rectification and ion selectivity. So far, heterogeneous membranes with 1D nanopores have been extensively studied. However, asymmetric structures with 3D micro-/nanoscale pore networks have never been investigated. Here, a simple and versatile approach to low-costly fabricate hydrogel/conducting polymer asymmetric heterogeneous membranes with electro-/pH-responsive 3D micro-/nanoscale ion channels is introduced. Due to the asymmetric heterojunctions between positively charged nanoporous polypyrrole (PPy) and negatively charged microscale porous hydrogel poly (acrylamide-co-acrylic acid) (P(AAm-co-AA)), the membrane can rectify ion transmembrane transport in response to both electro- and pH-stimuli. Numerical simulations based on coupled Poisson and Nernst-Plank equations are carried out to explain the ionic rectification mechanisms for the membranes. The membranes are not dependent on elaborately fabricated 1D ion channel substrates and hence can be facilely prepared in a low-cost and large-area way. The hybridization of hydrogel and conducting polymer offers a novel strategy for constructing low-cost, large-area and multifunctional membranes, expanding the tunable ionic rectification properties into macroscopic membranes with micro-/nanoscale pores, which would stimulate practical applications of the membranes.

The adaptability to wide salinities remains a big challenge for artificial nanofluidic systems, which plays a vital role in water–energy nexus science. Here, inspired by euryhaline fish, sandwich‐structured nanochannel systems are constructed to realize salinity self‐adaptive nanofluidic diodes, which lead to high‐performance salinity‐gradient power generators with low internal resistance. Adaptive to changing salinity, the pore morphology of one side of the nanochannel system switches from a 1D straight nanochannel (45 nm) to 3D network pores (1.9 nm pore size and ≈1013 pore density), along with three orders of magnitude change for charge density. Thus, the abundant surface charges and narrow pores render the membrane‐based osmotic power generator with power density up to 26.22 Wm−2. The salinity‐adaptive membrane solves the surface charge‐shielding problem caused by abundant mobile ions in high salinity and increases the overlapping degree of the electric double layer. The dynamic adaption process of the membrane to the hypersaline environment endows it with good salt endurance and stability. New routes for designing nanofluidic devices functionally adaptable to different salinities and building power generators with excellent salt endurance are demonstrated.

Both ion permeability and selectivity of membranes are crucial for nanofluidic behavior. However, it remains a long‐standing challenge for 2D materials to balance these two factors for osmotic energy harvesting. Herein, the MXene/metal–organic framework (MOF) hybrid membranes are reported to realize efficient ion‐permselective nanofluidic system, leading to high‐performance osmotic power generator. In the system, zeolitic imidazolate framework‐8 (ZIF‐8) is deposited onto the MXene surface and intercalated between the MXene nanosheets by electropolymerization approach. The angstrom‐sized windows of ZIF‐8 layer act as ion selectivity filters, endowing the membrane with high cation selectivity by size effect. The intercalation of ZIF‐8 crystals, reduces the interspacing of MXene, therefore, not only enhancing the ion permeability by shortening the ion transport pathway through the membrane, but also further improving the selectivity by increasing the overlap effect of electric‐double layer. The maximum power density reaches up to 48.05 W m−2 under 500‐fold salinity gradient with a high permeability (1263.3 A m−2), and a high selectivity of 0.906 at 50‐fold is obtained. This study provides a facile method to fabricate nanofluidic 2D membranes with both high ion permeability and selectivity for water nexus energy conversion.

Though decades have passed, the nanofluidic system that determines the RED approach process raises fundamental issues about the impact of surface charge on ionic transmembrane property. [5-8] Developments in nanomanufacturing have triggered technological revolutions in nanoporous membrane design and fabrication. [4,9-15] Selecting porous membranes is of the essence for the design of said energy devices. So far, inorganic composite, organic materials, soft matter hydrogel, [16] wood, [17] silk, [18,19] etc. [20-25] have been used to harness salinity gradient energy and have made a giant leap for the applications. Advanced polymers, especially functional ionomers, hold great potential in nanofluid devices because of their unique ion selectivity, uniform 3D pore structure, and physical mechanical properties. [26-28] Ionomers with available chemical diversity could self-assemble into 3D pores by intermolecular phase separation, which bear hydrophilic functional ionic pendants and hydrophobic backbone. [26-29] For decades, ionomers are extensively used as solid electrolytes in electrochemical technologies, especially serving as the Blue energy as a renewable, substantial energy resource has attracted scientists who are interested in discovering abundant membrane materials to achieve high power density. For decades, ionomers have been used as ion-exchange membrane to harvest this energy. Though extensive studies have been conducted, the underlying mechanism of ionic transmembrane behavior is still under debate. Here, the ionic transmembrane properties through membranes with 3D pores prepared by ionomers (polyphenylsulphone with pyridine pendants (PPSU-Py)) are systematically studied. A series of PPSU-Py with tunable porosities and surface charge densities is obtained simply by adjusting the percentages of the pendant. Nanoscale morphologies of the ionomers are simulated with the dissipative particle dynamic method, which is in agreement with the experimental data. Then, nanofluidic behaviors of as-prepared porous membranes are studied, which exhibit anion selectivity, pH gating, and modulated transmembrane conductance. Furthermore, a series of salinity gradient power harvesters based on the ionomers are constructed, of which the output power density is improved by tuning the charge density with the maximum output power density that reaches up to 1.44 W m-2. The impact of the ionomer on nanofluidic behavior is systematically discussed, and it is believed this work will shed light on nanofluidic materials and blue energy generator design.

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Building a combined interface in a Li2S cathode-based battery by integrating SPEEK into the cathode and inserting a SWCNT/rGO interlayer develops a new strategy from the viewpoint of interface engineering to achieve a high-performance Li–S battery.